Mechanical seals provide sealing between rotating portions of a machine, e.g. a rotating shaft and another portion of the machine, e.g. a stationary machine housing. The sealing action takes place between a seal face and a seal seat, one of which rotates with the shaft while the other is stationary and attached to the machine housing. The seal seat is usually made of hard material, such as silicon carbide. The seal face is usually made of softer material, such as carbon. Therefore, as the seal face and the seal seat rotate with respect to each other while in contact, the seal face wears much faster than the seal seat. As the seal face wears over time, because either the seal face or the seal seat is axially movable and pressed axially by springs and/or fluid pressure, the seal face and the seal seat remain in contact.
There is, however, a limit to the seal face wear that the axial movement of the seal face or the seal seat can accommodate. In typical seals, this wear limit is usually between 0.04 inch (1 mm) to 0.2 inch (5 mm), depending on the seal size and its design details. Once this limit is exceeded, the mechanical seal can no longer provide sealing action and the sealed fluid will leak out of the machine through the failed seal. It is, therefore, desirable to know the extent of the wear of the seal face so that the mechanical seal can be repaired before it fails unexpectedly due to a worn seal face and disables the machine in which it is installed.
The rate at which a seal face wears depends on seal materials, speed of rotation, contact pressure, temperature, sealed fluid type, machine vibrations and other material-dependent and machine-dependent parameters and conditions. There are empirical methods for estimating the average wear rate of seal faces and they can be used to predict the average useful life of a seal. However, these are estimates of the average life and not accurate predictions of the life of a specific seal. Because of random effects that the empirical methods cannot account for, the actual useful life of two identical seals installed in identical machines can be very different.
As an example, several new identical mechanical seals could be installed in identical machines (e.g., pumps) and operated under identical conditions. If the estimated life of the mechanical seals was three years, and they were operated till the last one failed, the average life of the seals could be quite close to the predicted life of three years. However, industrial experience shows that the first seal failure could occur in less than one year. This unpredictability of the life of a specific seal face makes maintenance scheduling based on life prediction ineffective. If such scheduling was used in this pump example, it would have failed at preventing the unexpected failure of the seal that failed in less than one year. It could have also resulted in unnecessary replacement of all the seals after three years even if some could have lasted much longer.
Therefore, seal maintenance scheduling that is based on measuring the wear of the seal faces is preferred over scheduling based on life predictions. Unfortunately, the measurement of seal face wear is difficult. Seal faces are located inside machinery housings and are not easily accessed from outside. Additionally, production line machines usually rotate continuously for long periods of time and must be monitored while in operation, they often operate at high temperatures, and often involve corrosion and accumulated contamination that make accurate measurements difficult.
A simplistic and impractical method for monitoring seals is to insert a mechanical probe axially into the seal until it contacts a part of the seal that moves axially when the seal face wears, and measure the depth of insertion. The depth of insertion could be calibrated to indicate the wear level of the seal face. The main problem of this simplistic method is that the force required to assure that the probe is contacting the seal part that moves axially could disturb the seal face contact with the seal seat and the lubricating film between them, and could allow a fluid borne solid particle to enter the contact area between them. Even an extremely small particle that is harder than the seal face material would cause failure of the seal in a very short time compared to its expected life. An additional problem of this method is that it would have to be accurate to within about 0.02 inch (0.5 mm) which is difficult to achieve in the hostile environments where many seals are used.
Several methods for monitoring mechanical seals have been patented. Most of them fall into one of two categories. The first category includes methods that measure and analyze signals and physical quantities that are directly related to the state of the contact area between the seal face and the seal seat. U.S. Pat. No. 4,748,850 and U.S. Pat. No. 5,041,989 disclose methods that measure and analyze the acoustic emission generated by the sliding surfaces of the seal. Seal condition is determined by comparing the minimum, maximum and mean values of the signal to values that correspond to normal operation. The method of U.S. Pat. No. 6,065,345 also monitors and analyzes the acoustic emission from the sliding surfaces. Additionally, this method also measures the operating parameters of the monitored machine, such as temperature, pressure, power and flow rate, and adjusts the acoustic emission signal thresholds according to the operating condition.
U.S. Pat. No. 6,360,610 discloses a method that detects the collapse of the lubricating film between the seal faces of a mechanical seal. An ultrasonic transducer is placed behind one of the seal faces and used to produce ultrasonic shear waves which propagate toward the interface between the two seal faces. By monitoring the amplitudes of the waves transmitted through or reflected by the interface, the method detects film collapse and the degree of contact between the seal faces. U.S. Pat. No. 6,325,377 discloses a mechanical seal with a monitoring port in its housing and a detector assembly connected to the port. The detector assembly includes acceleration, temperature and pressure sensors, and a communication device. Condition of the seal is determined by analyzing sensor signals that the communication device transmits to a monitoring system.
None of the methods in this category have proven commercially feasible because of variety of problems. First, these methods are complex and require extensive testing before alarm threshold levels can be established. Second, the measured quantities can change over time due to operating condition changes and these changes are difficult to predict. Consequently, they may be erroneously interpreted as indications of seal failures. Third, it is difficult to determine whether changes in measured quantities are due to a seal or due to other machine components. And fourth, there is a significant level of randomness in signals generated or affected by mechanical seals so that two identical seals mounted in identical machines may produce very different signals. Consequently, none of these methods is a commercially feasible solution to the problem of monitoring mechanical seals that are installed in industrial machinery.
The second category of patented methods for monitoring mechanical seals includes those that measure the movement of a seal component that moves axially to compensate for the wear of a seal face. U.S. Pat. No. 5,540,448 discloses a method where the movement of the seal component either connects or disconnects electrically two electrodes, thus indicating that the motion has reached a certain level. U.S. Pat. No. 6,595,523 extends this method and considers motion level measurement via conductivity test of either electrical, or optical, or sonic circuits. U.S. Pat. No. 4,424,973 discloses a method where the sealing face has a weakened area and when the seal face wear reaches the depth of the weakened area a minor leak develops that is detected. The seal can then be repaired before the wear reaches a level when it could cause a major leak. U.S. Pat. No. 4,497,493 discloses a method where a radially positioned optical or magnetic sensor measures the radial distance to an axially movable seal face member. The seal face member is shaped so that its axial movement results in a change of its radius at the location where the sensor is installed. The radial distance measured by the sensor is then translated to axial wear of the seal face based on the geometry of the seal face member.
U.S. Pat. No. 4,501,429 proposes to insert a fiber optic device into the mechanical seal. A seal element carrier is provided with indicia and the optic device is focused on the indicia. One can then determine visually how much the seal face moved because of face wear. It is not explained how this method would operate with fluids that are not transparent and with fluids that deposit contamination on the optic device and on the indicia. U.S. Pat. No. 7,280,219 and U.S. Pat. No. 6,580,511 disclose methods for monitoring seals using fiber-optic technology. Optical fibers embedded within the seal facilitate measurement of pressure, temperature and wear of the seal with an apparatus that uses light sources and an interferometric system.
The hostile environments in which most mechanical seals operate limits the practicality of seal face wear measurement techniques that use various electrical, magnetic, optical and sonic sensors for measuring distance. The sealed fluid is often electrically conductive, it affects magnetic fields, and it interferes with operation of optical and sonic sensors. Over time, deposits may accumulate on the sensors and change their characteristics. Magnetic, optical and sonic sensors have a finite sensing beam width and measure distance to an area that intersects the beam rather than to a point. This makes it difficult to measure accurately axial movement with a radially-positioned sensor. Another major problem is the calibration of such wear sensors. When a sensor is first installed, it can be calibrated to provide accurate measurement of the extent of wear of a seal face. However, mechanical seals wear slowly and the seal face wear may not reach a dangerous level until years later. The ability of these sensors to remain functional and remain calibrated for years while submerged in fluid and accumulating deposits is questionable. Consequently, these methods have not proven commercially feasible.
The present invention is a seal wear monitoring method and apparatus that are unaffected by the hostile environment in which seals operate. The present invention can measure accurately the wear of a seal face and it will do so even in hostile industrial environments. Additionally, the present invention does not require re-calibration during the life of the mechanical seal, it does not require electrical or optical wiring, and it does not require advanced technical skills from the user.